1802, from Modern Latin unilateralis, from unum, neuter of unus "one" (see one) + latus (genitive lateralis) "side" (see oblate (n.)). Jeremy Bentham (1748-1832) may have been the first to use it in the legal sense of "made or entered into by one party." Related: Unilaterally. Unilateral disarmament is recorded from 1929.

It is useless for the sheep to pass resolutions in favor of vegetarianism, while the wolf remains of a different opinion. [William Ralph Inge, "Outspoken Essays," 1919]